1. Field of the Invention
This invention relates to analytical apparatuses and methods for volatile organic compounds, and in particular, relates to air sampling and analysis utilizing a cryogenic concentrator and a gas chromatograph/mass spectrometer or gas chromatograph with multiple detectors.
2. Description of the Related Art
Environmental concerns about air pollution have led to the need for reliable, sensitive analytical methods for the identification of toxic or other undesirable volatile organic compounds (VOCs). Individuals, companies, and governmental entities at all levels often have a need to detect, identify and quantitate unknown or suspected air pollutants.
The use of canisters, TEDLAR.TM. bags and other standardized containers to sample air has proven to be an effective and efficient collection method in the analysis of air pollutants. The standard method is summarized in the EPA Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air (Method TO-14).
In Method TO-14, ambient air samples are collected in pre-evacuated SUMMA passivated canisters. After collection, the sample canister pressure may be above or below atmospheric pressure. Sample canisters are tagged with identification including pressure reading and transported to a specified site for analyses.
Before being analyzed, individual canisters at subatmospheric pressure may be pressurized to 20 psig with zero dry nitrogen. The resulting dilution is calculated based on the fixed volume of the can (i.e. six liters) and the pressure readings of the canister before and after the dilution with nitrogen.
Sample canisters are connected to the inlet of the analytical system. A pressurized sample canister may require a split of an established flow from the canister to deliver a specified mass flow controlled rate (i.e. 40 ml/min) to the concentrating section of the analytical system. Water vapor is reduced in the gas stream by a NAFION.TM. dryer. The VOCs are selectively concentrated by condensation in a reduced temperature (cryogenic) trap, while the main components of air (nitrogen and oxygen) are not retained.
Once the pre-determined volume of sample is concentrated on the cryogenic trap, the trap is rapidly heated. The VOCs are revolatilized and introduced to a GC column through a heated transfer line. The gas chromatographic capillary column recommended in Method TO-14 is cooled to a sub-ambient temperature (i.e. -50.degree. C.) to focus (minimize the volume of) the VOCs. An appropriate GC temperature program heats the GC oven containing the capillary column used to separate the sample components. Each component is detected by one or more detectors for identification and quantitation.
The canisters used for sampling atmospheric gases are generally made of stainless steel and have a volume of six liters at atmospheric pressure. Each sample and control requires a separate canister. After the sample collecting is accomplished, the canisters are typically piled in boxes or individually carried back to the site of the cryogenic concentrator and GC/MS in the laboratory. The canisters are individually connected in sequence to an inlet port on a multiposition valve to begin the processing. As is readily understood, the transport and repeated individual connecting are cumbersome and time-consuming operations.
It is therefore an object of this invention to provide a mobile device on which a plurality of canisters may be placed and transported.
It is a further object of this invention to provide a device having sample lines for a plurality of canisters to be connected to a multiposition controllable selective outlet.
It is a further object of this invention to provide a device and method enabling sample chain-of-custody determination. Other objects and advantages will be more fully apparent from the following disclosure and appended claims.